1. Field of the Invention
This invention relates in general to wellhead completion procedures, in particular to a method of installing a tubing hanger and internal tree cap in a wellhead using pressure assist methods.
2. Description of the Prior Art
In a typical offshore wellhead installation, upper and lower casing hangers will be suspended in the wellhead housing, each connected to a string of casing. Casing hanger seals will seal the annulus around each casing hanger to the bore of the wellhead housing. Tubing is lowered through the smaller diameter string of casing and supported by a tubing hanger. The tubing hanger in one type of installation lands and seals to the wellhead housing, using various types of running tools and seals. A production tree then lands on the wellhead housing for controlling through valves production fluids produced through the tubing. The production tree may be subsea or at the surface and connected by a tieback connection to a wellhead at the sea floor.
In another and recent type of offshore installation, known as a horizontal tree, the production tree is landed on the wellhead housing before the tubing hanger is installed. The tubing hanger is lowered into the tree and seals in the bore of the production tree. The tubing hanger and production tree have lateral flow passages which register with each other for producing the production fluids. A crown plug is installed in the tubing hanger and a tree cap installed on the tree. The horizontal production tree may be located subsea or at the production platform. Various types of seals are employed to seal between the tubing hanger and the bore of the production tree and tree cap and production tree. Various running tools are employed to install the tubing hanger, seals and tree cap.
Metal seals are desirable for offshore wellheads in general because of the longer life as compared to elastomeric seals. Metal seals, however, require a much higher setting force. Metal seals in general are normally set in offshore wellheads by using a hydraulic running tool. The hydraulic running tool is lowered on drill pipe through a blowout preventer which mounts to the wellhead. The hydraulic running tool may receive hydraulic fluid through a separate line for actuating a piston to set the metal seal. Alternately, the hydraulic running tool may be operated by pressure supplied to the annulus below the blowout preventer.
In one type of running tool used for running casing hanger metal annulus seals, the running tool has a bulk elastomeric seal that seals in the wellhead housing. The blowout preventer is closed on the drill pipe. Hydraulic pressure is supplied down a choke and kill line to the annulus around the drill pipe between the blowout preventer and the bulk seal. The running tool has a piston that causes setting of the casing hanger seal. While successful, these hydraulically actuated running tools for supplying forces for setting metal seals are fairly complex and expensive.